The tumor suppressor p53 is the most commonly mutated gene found in human cancers [3]. It plays a vital role in guarding mammalian cells against tumorigenesis [22,23]. The Cdc25C phosphatase plays an important role in cell cycle progression at the G2/M transition [7-10]. Its activity is inhibited in response to DNA damage thereby contributing to the G2 checkpoint response [7-10]. Cdc25C is a target for transcriptional repression by p53. Overexpression of Cdc25C sensitizes cancer cells to DNA damage induced cell death [11, 12]. Interestingly, normal untransformed cells are not sensitized to this treatment [12]. Therefore, Cdc25C repression inhibition could lead to a novel therapeutic approach in combination with DNA damage. In order to address this possibility, a clear molecular basis for p53-dependent repression of Cdc25C must be described. As of yet, three distinct mechanisms of repression have been proposed, one involving direct binding to DNA by p53 and two indirect involving different elements. Each of which may provide molecular targets for inhibition. The present study aims to dissect the molecular basis for p53-dependent repression of Cdc25C and to explore its relevance as a tool that can be exploited in order to enhance chemotherapeutic responses in cancer patients. Hypothesis: It is hypothesized that the p53 response element is required in order for p53 to bind to the Cdc25C promoter and for the recruitment of co-repressors. Additional factors, such as p21, may be required for this repression. Furthermore, inhibition of this mechanism in cancer cells can abrogate DNA damage-induced Cdc25C repression, which results in heightened sensitivity to DNA damage-induced cell death. Specific Aims: Aim 1: Analysis of the molecular basis for DNA damage-induced p53-dependent repression of Cdc25C. A) Characterize the role of Histone Deacetylases (HDACs) and DNA methyltransferases (DNMTs) in Cdc25C repression. B) Study the role of CCAAT box binding factor, NF-Y in p53-dependent repression of Cdc25C. Aim 2: Functional analysis of different elements in the Cdc25C promoter. A) Characterize relevant elements in the Cdc25C promoter in order to identify potential therapeutic targets. B) Explore methods to use Cdc25C repression inhibition as a novel therapeutic approach. Aim 3: Study of the role of p21 in the p53-induced repression of Cdc25C. A) Investigate the p21 dependence of p53 and co-repressor recruitment to the Cdc25C promoter. B) Study the role of Rb family proteins in Cdc25C mediated repression in response to DNA damage. PUBLIC HEALTH RELEVANCE: The present study aims to dissect the molecular basis for p53-dependent repression of Cdc25C and to explore its relevance as a tool that can be exploited in order to enhance chemotherapeutic responses in cancer patients.